This invention relates to the production of a unique class of granular graphitic carbon materials containing metallic elements dispersed throughout the graphitic structure. More specifically, the unique class of products contain elements that enhance the controlled nucleation of dissolved carbon during the solidification of molten cast irons and are of particular value in the manufacture of quality iron castings. Moreover, the chemical form of the selected metallic elements and their high degree of dissemination within and throughout a graphitic carbon structure has been shown to provide special properties that increase the controlled nucleating or "inoculating" ability of the products of this invention over that obtained by use of the components added individually or in a simple mixture to molten cast iron.
Commercial cast irons are iron alloys containing 2-4 percent dissolved carbon and 1-3 percent silicon. On cooling and solidification of the molten iron, the presence of silicon causes the dissolved carbon to precipitate as a crystalline form of graphite rather than the brittle iron carbide Fe.sub.3 C (Cementite).
The form in which the graphite precipitates from the melt is extremely important to the type and quality of the iron castings made. In "grey" iron castings, the graphite has been precipitated in the form of flakes; whereas in nodular or ductile iron, the graphite has precipitated in spherical form. As a result of the different morphologies of the precipitated graphite structure, the iron castings obtained have very substantially different properties and commercial uses. For example, grey cast irons have relatively poor mechanical properties; however, "nodular" iron has excellent mechanical working characteristics and is finding increasing markets.
One key factor in determining the shape and size of the graphite formed is the rate of graphite precipitation. Liquid iron can be cooled to well below the equilibrium temperature for crystallization of graphite and thus be "super-cooled" relative to its carbon content in a manner analgous to the super-cooling of aqueous salt solutions. A high degree of super-cooling, also termined "undercooling" is undesirable in that precipitation of cementite occurs before the graphite precipitates. The cementite is extremely brittle and the casting is very difficult to machine and has relatively poor strength. Moreover, molten cast iron that is supercooled is in a thermodynamically unstable condition and once nucleated, graphite precipitation can be extremely rapid and the ability to control either the size or shape of the precipitated graphite is lost.
Much of the development of modern iron foundry technology relates to the understanding and control of the precipitation of graphite during solidification of molten cast irons. Present practice employs the addition of inoculant materials to the melt. These inoculants often are ferro-silicon base alloys containing other elements in solid solution which serve to nucleate the graphite precipitation and thus prevent "undercooling" or excessive "chill". In addition to "seeding" the precipitation of graphite from the melt, certain elements used in the inoculants are also able to influence the characteristic shape of the graphite formed. As noted, the properties of the resulting casting are directly related to the degree of graphite precipitation and the size and shape of the graphite. With the increasing need for higher strength, lighter, thinner wall castings, the control requirements are continually becoming more rigid and metallurgists are seeking better methods and additive materials to improve their ability to control the cast iron process.
The addition of graphite carbon to molten iron before casting is a well known means to promote the nucleation of graphite from the melt and thereby prevent undercooling during solidification. It is recognized that the carbon additive must have a high degree of graphite crystallinity in order to provide appropriate "nucleation sites" for precipitation of graphite from the melt. Non-graphite carbons such as petroleum coke, metallurgical coke or baked carbon scrap are not effective inoculants. Graphite materials that are available for this purpose are either natural mineral graphite or synthetic graphite such as electrode scrap graphite.
Despite the recognized ability of crystalline graphite to serve as an inoculant, the use of graphite has several disadvantages. First, the mineral graphites that are available in large tonnage and reasonable cost contain ash forming components as impurities. These gangue materials comprise various silicate and clay minerals which can vary considerably in amount and composition even from the same mineral deposit and their use leads to problems in establishing uniform addition of graphite. More importantly, the gangue materials are slag formers and can easily upset the control of the fluidity of the slag and thereby create difficult problems in handling and transfer of the hot metal. The use of synthetic graphite, although a highly pure form of graphite with essentially no slag forming constituents, is readily dissolved into the melt. The extremely rapid dissolution of this form of graphite into the molten iron rapidly depletes the melt of nucleation sites and the effectiveness of synthetic graphite "fades" too rapidly to be entirely effective. Moreover, synthetic graphite does not contain elements that serve as deoxidizers and desulfurizers such as aluminum, calcium or magnesium or other elements such as those of the rare-earth group of elements which are commonly used in foundry practice to increase the tensile strength, provide nucleation sites, and to promote the formation of spherical graphite as required to make nodular iron. Thus despite the ability of graphite materials to inoculate cast iron compositions, graphite is rarely used alone. The foundry operators therefore must use various mixtures of materials that are generally not available from a single supplier and often not made specifically for foundry application and must be mixed at the foundry leading to undesired auxiliary operations, more possibilities for material losses, furnace control problems and generally decreased operating efficiencies. Therefore, significant improvement in the efficiency foundry operation would be possible if inoculant materials were available having the appropriate sizing and carefully controlled and chemically combined active elements that could be used as single additions to achieve the desired control of the graphite precipitation and the resulting properties wanted in the cast iron products being made.
It is the objective of the process and products of this invention to produce a graphitic carbon material that will serve as effective inoculants to allow the controlled precipitation of graphite from molten cast irons during solidification. It is a further objective to provide a graphitic carbon material that when added to molten cast iron yields nucleation sites that can be retained within the melt for a sufficient period to achieve uniform castings without need for repeated inoculation. Additionally, it is the objective of this invention to produce materials that enhance the precipitation of graphite in spherical or nodular form. It is a further objectives of this invention to achieve a granular addition product that can be readily handled by foundry operators without generating dusts and which contain all active ingredients in chemically combined form that will be used with high metallurgical efficiency. And finally, it is the objective of this invention to produce such inoculant products in a continuous, energy efficient manner that is carefully controlled and environmentally acceptable.